1. Field of the Invention
This invention relates to merocyanine dyes comprising rhodanine, barbituric acid, thiobarbituric acid, and isoxazolone groups, condensation polymers and (meth)acrylated polymers incorporating such dyes, the dyes and polymers being useful in nonlinear-optical applications.
2. Description of the Related Art
Several polymers have been reported in the literature for use as active nonlinear-optical (NLO) compositions, see for example Nonlinear-optical Properties of Organic Material 1147 (Khanarian, G., ed., SPIE Proc., San Diego, 1989). Most of them are linear polymers bearing NLO-active groups (hereinafter for brevity referred to as "nonlineaphores") as side chains, in which the aligned dipoles tend to relax over extended periods of time at room temperature or slightly elevated temperatures. Some of the noted exceptions are the acrylates/methacrylates reported by Man et al. Proc. SPIE-Intl. Soc'y Opt. Eng. 1213 (Photopolym. Device Phys., Chem., Appl., 1990); and polyurethanes reported in U.S. Pat. No. 5,001,209 (Wreesman et al.). These polymers have glass transition temperatures (T.sub.g), above 135.degree. C. and are reported to be stable at elevated temperatures.
Recently, an increasing number of publications have appeared in which the issue of elevated temperature stability is being addressed by use of cross-linked polymers. These polymers can be divided into two classes, namely, those cross-linked around the NLO units, to restrict the free volume available for relaxation of the NLO moiety, and those cross-linked through the NLO units, to covalently bind the NLO moiety of the NLO unit at more than one point.
U.S. Pat. No. 4,886,339 (Scozzafava et al.) describes one example of the former case. The disadvantage of this system is that a binder is needed to make a film of the monomer prior to poling/cross-linking and the reaction has to be carried out in an inert atmosphere. Furthermore, there is no guarantee that the resulting polymer will have a high enough cross-link density to prevent all rotation of the dipoles since the molecule is covalently bound only at one point. As reported by Park et al. Chem. Mater. 2 229 (1990), this type of cross-linking does provide some enhanced stability of the NLO moiety over versions that are not cross-linked but long term relaxation is still a problem.
Eich et al. J. Appl. Phys. 66 3241 (1989), have addressed the relaxation problem by utilizing epoxide chemistry to produce stable systems in which the NLO units are covalently held at more than one point. They report stability up to 85.degree. C. for a film corona-poled at 140.degree. C. for 16 hours. A possible problem of this system, apart from the long curing time required, is the long-term dimensional instability of the polymer film, especially in high humidity, since amine cured epoxides are known to have an affinity for water.
The phenomenon of molecular hyperpolarizability and related NLO effects are described in ACS Symposium Series 233, Am. Chem. Soc'y, (Washington, D.C. 1983). Generally, hyperpolarizable molecules have a delocalized pi (.pi.)-electron system in which an electron donor group and an electron acceptor group are conjugatively coupled directly by a .pi.-electron system. Molecules possessing large molecular hyperpolarizability are capable of ready polar orientation in an electric field. As a result, the affected material also becomes macroscopically hyperpolarizable. Such a material may be used in an optical switch in which the material is dispersed in a polymer host and an electric field is applied to the hyperpolarizable polymer. Such a use is described by Thackera Appl. Phys. Lett. 52 1031 (1988).
Heterocyclic dyes containing a hemioxonol nucleus were disclosed in U.S. Pat. No. 2,956,881 (Herold et al.). These dyes were used to alter the sensitivity of photographic emulsions.
More recently, Japanese Patent No. JP 2,179,624 (Ikeda et al.) discloses straight chain polyene compounds (general structure shown below) utilizing hemioxonol-based electron acceptor groups in NLO applications; ##STR1## wherein R, R.sup.1 and R.sup.2 are independently a C.sub.1-3 alkyl group, or R.sup.1 and R.sup.2 taken together form a C.sub.4-6 polymethylene group or --CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 --; X is a heteroatom, wherein such heteroatom is selected from the group consisting of O, Si, Se, or X is NC(0)R.sup.3, wherein R.sup.3 is a C.sub.1-3 alkyl group; Y is a heteroatom, wherein such heteroatom is selected from the group consisting of O, S, or Se; and n is an integer of 1 to 4.
The compounds described in JP 2,179,624 (Ikeda et al.), though they may possess large molecular hyperpolarizability, would be susceptible to chemical degradations, such as photochemical degradation or oxidation. The alternating double bonds in the polyenes tend to undergo oxidation similar to the known oxidation of retinol (Vitamin A) and its derivatives, see for example, Kirk-Othmer Encycl. Chem. Technol 21 494 (2 ed.).
Ikeda et al. Chem. Lett. 1803 (1989), have disclosed the use of dyes based on barbituric acid, hydantoin and rhodanine as small molecules with large second order hyperpolarizabilities. However, no mention is made of how to derivatize these molecules for polymer synthesis, thereby enabling incorporation of these molecules into either linear or cross-linked polymers. Furthermore, no mention is made of how to proces these materials into thermally stable, optically clear NLO polymeric films.
Mandal et al. Makrom. Chem. Rapid Commun. 12 63 (1991) have also described a photo-crosslinked material for NLO. In this material, the cross-links are formed by photo-dimerization of cinnamoyl groups. Drawbacks of the system are that only a modest concentration of NLO nonlineaphores (.about.20%) was achieved, and the reported second harmonic generation (SHG) temperature stability is limited to 65.degree. to 80.degree. C.
Cross-linked sulfone acrylates and methacrylates have been described in U.S. Pat. No. 4,796,971 (Robella et al.). However, the examples describe synthesis of materials in which the nonlineaphore is bound to the polymer network at only one end of the extended .pi.-electron system. The NLO molecule is covalently bonded to the polymer at one point of attachment. As reported by Park et al., supra, this type of cross-linking does provide some enhanced stability of the NLO moiety over uncross-linked versions, but long term relaxation is still a problem. The material that was poled was a neat, functionalized nonlineaphore, or a mixture of functionalized nonlineaphore and a polymer binder. The polymer binder increased the viscosity of solutions, allowing thicker films to be produced, and improved the optical quality of the films by reducing the crystallization of the NLO component. However, the presence of the polymer binder reduced the nonlineaphore density in the final NLO material.
European Patent Application 0 445 854 A1 (van der Horst et al.) describes a thermally curable system containing donor conjugated .pi.-electron acceptor (D.pi.A) groups that can be poled by an electrical field while, simultaneously, the system is cured. The disclosed material comprised a D.pi.A group containing compound (A) having two or more functional groups and a compound (B) having two or more functional groups reactive towards (A). One of the components (A) or (B) contained isocyanate groups. To obtain a polymeric network, at least one of the components (A) or (B) should be tri-functional or higher. The NLO molecule is covalently bonded at one point of attachment. As reported by Park et al., supra, this type of cross-linking does provide some enhanced instability of the NLO moiety over uncross-linked versions, but long term relaxation is still a problem.
As reported by Dai et al. Materials for Nonlinear Optics, ACS Symposium Series 455 pp 226-49 (Marder et al. ed 1991), NLO-active molecules can have functionality at both the donor and acceptor ends. The below-illustrated molecule was combined with epoxides to make crosslinked NLO polymers. The disadvantage of this type of functionality, that is, two identical groups, is that attachement of the nonlineaphore to the matric and crosslinking occur in the same step. Therefore, the prepolymerization step used to allow film formation (or avoid dielectric breakdown) would result in immobilization of some nonlineaphores in random configurations. Also, high nonlinaphore concentrations were not achieved in this system.